diff --git a/ni845x_spi.c b/ni845x_spi.c
index 21d39a8..00a9f11 100644
--- a/ni845x_spi.c
+++ b/ni845x_spi.c
@@ -169,7 +169,7 @@
 
 		device_pid = pid;
 
-		if (!serial || strtol(serial, NULL, 16) == serial_as_number)
+		if (!serial || strtoul(serial, NULL, 16) == serial_as_number)
 			break;
 
 		if (found_devices_count > 1) {